1. Field of the Invention
The present invention relates to a persistent-mode superconducting magnet apparatus in a persistent-current operation, which is incorporated, for example, in a physical and chemical NMR (nuclear magnetic resonance) analyzer or a medical magnetic resonance imaging device (MRI).
2. Description of the Related Art
In general, such a physical and chemical NMR analyzer requires a persistent-mode superconducting magnet apparatus having a high strength of magnetic field and an extremely low temporal change (decay) of magnetic field. Hence, the persistent-mode superconducting magnet apparatus generally uses a closed circuit formed by using a persistent-current switch for a persistent-current operation.
Indeed, however, during a persistent-current operation, mainly due to the minute joint resistance at joints of the superconducting wires between the superconducting coils, a persistent current gradually decays over time to produce a magnetic field decay. A normal soldering at the joints of the superconducting wires makes it difficult to decrease the joint resistance to about 10xe2x88x929 ohm or lower since soldering materials are not superconducting materials; however, a joint technique has been developed in which less connectable filaments of the superconducting wires are directly connected by spot welding, pressure, or the like to maintain it at about 10xe2x88x9212 ohm. This can realize a persistent-mode superconducting magnet apparatus having a magnetic field decay rate of approximately 0.01 ppm/hr.
However, such a joint resistance value between superconducting wires is affected by a magnetic field. Once the magnetic flux density of that joint reaches about 1 T (tesla=104 gausses) or more, the joint is transitioned from a superconducting state to a normal metallic conducting state, and the joint resistance value is sharply increased to provide a higher electric current (magnetic field) decay. Hence, a specific action must be taken such that the joint is magnetically shielded. As a method of providing an extremely stable magnetic field without such a specific action, a method disclosed in Japanese Examined Patent Application Publication No. 4-61103 has been proposed heretofore.
In FIG. 7, a persistent-mode superconducting magnet apparatus 30 according to this method includes a first superconducting magnet 31 on the outer peripheral side, and a second superconducting magnet 32 on the inner peripheral side, which are concentric with each other, inside of which a working space is formed. On the outer peripheral side of the first superconducting magnet 31, field-correcting superconducting coils 33 are further disposed in a concentric manner with the first superconducting magnet 31 and the second superconducting magnet 32.
The first superconducting magnet 31 includes a first superconducting coil 34 which is cylindrically wound, a first persistent-current switch 35 connected in parallel to the first superconducting coil 34, an energizing (field exciting) power supply 36 for supplying an electric current to the first superconducting coil 34, and a heater power supply 37 for supplying an electric current to a heater of the first persistent-current switch 35. The second superconducting magnet 32 includes a second superconducting coil 38 having another superconducting wire concentrically and cylindrically wound on the inner periphery of the first superconducting coil 34, a second persistent-current switch 39 connected in parallel to the second superconducting coil 38, an energizing (field exciting) power supply 40 for supplying an electric current to the second superconducting coil 38, and a heater power supply 41 for supplying an electric current to a heater of the second persistent-current switch 39.
Within a cryostat 42, the first superconducting magnet 31 and the second superconducting magnet 32 are electrically independent from each other, and the second superconducting coil 38 is provided on the inside of a bore of the first superconducting coil 34. Accordingly, the magnetic field decay in the working space along with the electric current decay of the second superconducting coil 38 is compensated by an increment in the magnetic field of the working space in the first superconducting coil 34, which is caused by the electric current mutually induced by the first superconducting coil 34 along with that electric current decay, so that the magnetic field of the working space attempts to be maintained in an extremely stable manner. In other words, making a reduction in the magnetic field, which is caused by an electric current of the second superconducting coil 38, to be equal to an increment in the magnetic field induced therefrom, which is caused by an electric current of the first superconducting coil 34, may maintain a constant field strength in the working space.
Meanwhile, the aforementioned conventional approach requires the separate and independent energizing power supplies 36 and 40, as well as the heater power supplies 37 and 41 for the separate and independent persistent-current switches 35 and 39, in order to energize the two independent superconducting magnets 31 and 32, respectively. In addition, when the first superconducting coil 34 and the second superconducting coil 38, which are electrically independent from each other, are energized, there is a probability that forces acting on the coils in the magnetic field are not balanced resulting in damage to the coil windings, and thus the superconducting magnets 31 and 32 must be concurrently energized at the same proportion of energizing rate. Accordingly, since two energizing arrangements such as the energizing power supplies 36 and 40 are employed and the two arrangements are operated, the operation of the superconducting magnet apparatus 30 is complex, and because of complexity, this operation is considerably different from the operation of a persistent-mode superconducting magnet apparatus having only one superconducting magnet (the case where there is no need for concurrent energizing at the same proportion of energizing rate).
The present invention overcomes the foregoing conventional problems, and has an object to provide a persistent-mode superconducting magnet apparatus which yields an extremely stable central magnetic field in the vicinity of the magnet apparatus center without any specific technique or action on the superconducting joints and by using similar energizing equipment and operation to those of a magnet apparatus having only one superconducting magnet.
A persistent-mode superconducting magnet apparatus according to the present invention is characterized by including: a superconducting magnet having a plurality of unit superconducting coils connected in series; a first persistent-current switch connected to the ends of the series circuit formed of the unit superconducting coils connected in series; and at least one second persistent-current switch connected to the ends of any one or a predetermined number of consecutive unit superconducting coils of the plurality of unit superconducting coils. In this case, a plurality of second persistent-current switches may be disposed.
With this construction, a superconducting magnet circuit includes, besides a closed circuit formed of a plurality of unit superconducting coils connected in series and a first persistent-current switch, at least one second persistent-current switch which forms a closed circuit by connecting the ends of any one or a plurality of unit superconducting coils of the plurality of unit superconducting coils, and is thus divided into at least two closed circuits, whereby their mutual induction may suppress the magnetic field decay in the vicinity of the magnet apparatus center, thus maintaining the magnetic field in the vicinity of the magnet apparatus center in an extremely stable manner. Furthermore, each of the superconducting magnets which form at least two closed circuits has a plurality of unit superconducting coils connected in series, and can be easily energized concurrently at the same proportion of energizing rate, whereby there is no need to use two energizing arrangements such as the usual use of energizing power supplies for two electrically independent superconducting magnets, thus reducing the number of parts. Further, there is no need to operate the two arrangements, thus preventing the operation from being complex. Therefore, it can be easily energized concurrently at the same proportion of energizing rate without any specific joint technique such as spot welding or any specific action such as magnetic shielding for the joint between superconducting wires, and by using similar energizing equipment (a single energizing power supply and a single heater power supply) to that of the one having only one superconducting magnet, as well as by using similar operation to that of the one having only one superconducting magnet, thereby preventing the operation from being complex as usual, providing an extremely stable central magnetic field in the vicinity of the magnet apparatus center.